The present invention relates to a printhead and a printing apparatus using the same and, more particularly, to a printhead having an element board or a substrate on which a digital circuit including a printing element and a driving means for driving the printing element in accordance with input printing data, and an analog circuit including a sensing means for sensing information about printing (information about the board/substrate state) are formed by a semiconductor process, and a printing apparatus using the same.
Note that the present invention is applicable not only to a general printing apparatus but also to a copying machine, a facsimile apparatus having a communication system, a word processor having a printing unit, and an industrial printing apparatus combined with various processors.
In a conventional inkjet printing apparatus using thermal energy, the electrothermal transducer (heater) of a mounted printhead, and a driving circuit for the electrothermal transducer are formed on the same board using a semiconductor process technique, as disclosed in, e.g., Japanese Patent Laid-Open No. 5-185594. It is also proposed to form, on the same board, elements for sensing the state of this board, e.g., the distribution state of the board temperature or resistance value or variations in the characteristics of the driving circuit.
Outputs from these sensing elements are often analog signals, and a signal processed in a signal processing circuit for feeding back a driving signal using these outputs is often an analog signal. Such an analog signal is readily influenced by noise. To solve this problem, Japanese Patent Application No. 11-198095 discloses an arrangement of converting analog signals into digital values by an analog-to-digital conversion means formed on the same board, and supplying the digital values to the signal processing circuit to optimize the driving signal.
In any case, a circuit block for processing an analog signal exists on the board. A circuit for sensing a small signal level or detecting a signal at a high resolution may be mounted. On the board on which the heaters and driving circuit used for the printhead are integrally formed, circuit blocks which operate by digital signals, such as a shift register for temporarily storing image data to be printed, a latch circuit for latching the image data, and a decoder circuit for sequentially selecting heaters to be driven are also formed by the same process. These digital circuits receive clock signal pulses serving as the operation references.
More specifically, an analog signal processing circuit for sensing the board state and a digital signal processing circuit for driving the heater in accordance with image data exist on the same board of the printhead. The printing speed required for a printhead and printing apparatus is increasing year by year. Along with this, the image data transfer clock frequency is also increasing. As the printing speed increases, the driving frequency of a heater for generating heat for performing printing is increasing. This increases the heat generation amount per unit time, and increases the temperature of the entire board. Thus, the temperature rise of the board must be sensed to feed back driving operation at higher precision.
As the clock frequency increases, the analog circuit-system for sensing the temperature at high precision may malfunction under the influence of radiation noise or line noise generated from the digital circuit system. To prevent this, there has conventionally been proposed an arrangement of applying the voltages of analog and digital systems from separate power supplies to suppress mixture of noise. In this case, the voltages of the analog and digital systems are applied to circuits on a board via different power supply lines and power supply terminals.
In this prior art, however, two power supply paths exist at positions close to the printhead. If the power supply line is short, noise can be greatly reduced. To the contrary, if the power supply line extending from the power supply to a power supply terminal formed on the printhead is long, radiation noise or coupling noise generated by a clock pulse supplied to the digital system is mixed in the signal of the analog system circuit owing to the inductance component of the power supply line or the capacitance component between the power supply line and another line. As a result, the analog system decreases in precision or malfunctions.
The separate power supplies must be arranged for the analog and digital circuit systems when the voltages of the two systems are different. When, however, these voltages are the same (e.g., 5 V), the same voltage is repetitively applied from the separate power supplies.
It is an object of the present invention to provide a printhead in which mixture of noise between the power supplies of analog and digital systems is suppressed even when a power supply line, such as a flexible cable or printed substrate wire, is long, and a circuit for processing a small analog signal, e.g., a temperature sensing circuit, does not decrease in precision even for a high clock frequency, and a printing apparatus using the same.
It is another object of the present invention to provide a printhead capable of suppressing mixture of noise in the circuit systems while sharing a power supply when the power supply voltages of the analog and digital systems are the same, and a printing apparatus using the same.
To achieve the above objects, a printhead according to the present invention comprises an element substrate on which a digital circuit including a printing element and driving means for driving the printing element in accordance with input printing data, and an analog circuit including sensing means for sensing information about a state of the element substrate are formed by a semiconductor process, and an electrical circuit substrate which is connected to the element substrate, and has a plurality of external input terminals for inputting/outputting signals from/to the element substrate, wherein a power supply of the digital circuit and a power supply of the analog circuit are commonly connected to the other terminal of a capacitor having one terminal grounded on the electrical circuit substrate.
The above objects can also be achieved by a printing apparatus for performing printing using the above printhead.
More specifically, in a printhead in which a digital circuit including a printing element and driving means for driving the printing element in accordance with input printing data, and an analog circuit including sensing means for sensing information about printing are formed on the same element substrate by a semiconductor process, and which comprises an electrical circuit substrate having a plurality of external input terminals for inputting/outputting signals from/to the element substrate, the power supplies of the digital and analog circuits are commonly connected to the other terminal of the capacitor having one terminal grounded on the electrical circuit substrate.
This arrangement can avoid mixture of noise of the digital circuit in the analog circuit, and decrease the noise level.
The operation precision of the analog circuit such as a substrate temperature sensing circuit or heater resistance value monitoring circuit increases. The printing quality of the printhead can increase, and malfunction can be prevented, resulting in high performance.
In this case, the power supplies of the digital and analog circuits are arranged apart enough not to transmit the influence of noise, which can suppress mixture of noise generated inside the element substrate.
The digital circuit preferably includes a shift register for temporarily storing the printing data and a latch for latching the data stored in the shift register.
The analog circuit preferably includes either one of means for sensing a temperature outside the element substrate and means for monitoring a heater resistance value.
The capacitor preferably has a capacitance of 0.1 xcexcF to 10 xcexcF.